Analog electronic signals can be represented or reconstructed by a certain set of discrete sampled values or samples. Many electronic applications use samples of analog signals instead of the analog signals. For example, digital data processing and communication systems use digital data converted from samples of an analog signal to achieve improved noise immunity and processing flexibility in data processing and transmission. Conversion of analog signals to digital data can be accomplished by first sampling the analog signals into sampled values and then digitizing the sampled values in a desired form.
A sampling rate or sampling frequency describes the number of samples taken from an analog signal per unit time (e.g., one second). The minimum sampling rate should be equal to or greater than the Nyquist rate, which is double the highest frequency in an analog signal, in order to preserve the minimum information content in the original analog signal. Thus, a high sampling rate is desirable in converting an analog signal with signal components at high frequencies into digital form. In addition, an analog signal may be oversampled at a sampling rate much higher than the Nyquist rate to improve the signal-to-noise ratio and/or precision of a subsequent analog-to-digital conversion.
Sampling of analog electronic signals is usually accomplished electronically by using electronic circuits. Electronic sampling techniques and circuits are well developed. The maximum sampling rate achievable by an electronic circuit is generally limited by the response times of the electronic components and the circuit configuration. This further limits the conversion speeds of many electronic analog-to-digital converters.
Such speed limitation in electronic digital-to-analog conversion can be an obstacle to many applications that require high-speed analog-to-digital conversion. Real-time digital video in applications such as telecommunication and machine vision is one such example. The performance of the real-time digital video in the existing video delivery on the Internet and in video conferencing systems is not only limited by the bandwidth limitation in the data transmission channels but also limited by the analog-to-digital conversion rates.